A Q&A session on the Rockstar Games Website addresses the frequent requests they've received for a PC edition of Red Dead Redemption, saying this is still not happening:

We hope that with the announcement that L.A. Noire is coming to PC this fall, and the knowledge that our next big game Max Payne 3 will have a PC release (plus that we’ve released both Grand Theft Auto IV and the Episodes from Liberty City for PC), we can finally put to rest any misconceptions that we’ve ‘abandoned the PC platform’. We do know that, yes, there is just one title absent from our PC release plans – that game of course being Red Dead Redemption, and of course we’re well aware that some fans have been asking for it. All we can say is that whenever it is viable (technically, developmentally and business-wise) for us to release a game for PC (or any other particular platform) – we will and we usually do; unfortunately, that is just not the case 100% of the time for all platforms. To PC gamers, we hope that between the Grand Theft Auto titles, and the forthcoming L.A. Noire and Max Payne 3, you’ll have plenty of great Rockstar action to enjoy.

Creston wrote on Oct 6, 2011, 00:26:Well SHIT, Rockstar. The ONE frakking game of yours that SCREAMS for a PC version, both for the 9000000000000000000000000 times more powerful graphics and for being able to actually AIM, and you don't make a PC version of it.

I am disappoint.

Creston

Pretty much this. It makes not sense that they'd skip RDR and release stuff before and after it. It's almost like they're doing it on purpose to hurt me.

Odds are the engine was heavily modified and would take a bunch of work to get onto PCs. No other real reason why this would be the only one they skip.

Far and away their best game, in my opinion, but I absolutely loved this game and kind of hated GTA4, so...